Reducing the Risk of Greenwaste Fires

Spontaneous combustion is not a science fiction phenomenon, nor is it some type of urban legend. It is, instead, perhaps the most common ignition source for greenwaste fires at landfills.  Decomposition of greenwaste is a natural process. And when it occurs naturally, such as when leaves decompose in the forest, it is a very slow biological process.

And though the process is exothermic-meaning it gives off heat-in nature, the rate is typically so slow that the visual process and the heat it generates are imperceptible.

However, when you accumulate massive quantities of unprocessed greenwaste, the conditions become less natural and the process can be greatly accelerated-to the point that the greenwaste generates enough heat to set itself on fire, a process known as spontaneous combustion. There are generally three conditions that are required for spontaneous combustion to occur.

First, there must be enough green material to allow decomposition to occur very rapidly. Large quantities of wet or green leaves or grass clippings provide those areas within your stockpiled greenwaste that will get very hot, very quickly.

Second, there must be an adequate airflow. Porous materials such as branches and coarse vegetation can provide enough open structure to allow lots of airflow. Increasing airflow (i.e., more oxygen) will continue accelerating the decomposition process and building heat at the same time.

Third, the pile must be tall enough to allow the creation of a chimney effect, an isolated pathway through which the heated air exits through the pile. The chimney effect will focus the air intake on the hot area, similar to the way you’d blow on a spark to start a campfire.

Under ideal, perfect storm conditions, the heat will rise through one or more narrow confined passageways toward the top of the pile. In this case, the height of the pile also plays an important role.

As the heat rises, the pathway that it follows begins to act as a chimney, and the taller the chimney, the greater the flow.  In tall piles of greenwaste-15 feet, 20 feet, or even 25 feet tall-that “chimney effect” can become very pronounced. Like a fireplace chimney, as the airflow heats up the surrounding material, it creates additional draft.

In essence, it’s an increasing spiral effect: As the heat rises, it begins to draw air …The more air drawn into the system, the more the decomposition process accelerates …Which creates more heat, which …well, you get the idea.

Once the system gets to this point, it takes on a life of its own.

As the greenwaste decomposes and heats up, at some point around 180º Fahrenheit, the process begins to shift from a biological reaction to a chemical reaction. At these temperatures the bacteria that initially started the decomposition process will become dormant or die.

At this point, unless you do something to change the mix-break the pile apart, shut off the airflow, or add water-this process is well on its way toward spontaneous combustion.

It is essentially an accelerating oxidation process, meaning that the organic material in the greenwaste is combining with the oxygen and a chemical reaction.

This is exactly what a fire is. In a fire, oxygen is combined with carbon to create carbon dioxide and water. That’s the reaction of a fire.

So, how do you minimize the risk of a spontaneous combustion fire in your greenwaste pile?  Here are some key steps.

Don’t mix wet greenwaste with porous, coarse vegetation. Keep branches and coarse vegetation in one pile and wet, green material in another. The branches and coarse vegetation without the presence of greenwaste simply won’t decompose fast enough to create the temperature needed for spontaneous combustion.

Similarly, even though the greenwaste could decompose rapidly, without the porous material it generally will not receive enough air to accelerate the decomposition process for the rate required for a spontaneous combustion.

Maintain the total depth of your greenwaste stockpile to 10 feet or less, and you’ll reduce the likelihood of a chimney forming that can provide a focused draft to accelerate to spontaneous combustion. As the depth increases, the chance of fire increases to the point that piles of mixed vegetation that are 20 to 25 feet tall have a very high likelihood of catching fire.

Inspect your greenwaste pile daily-early morning is best. Look for vents that would indicate a chimney. These will be openings near the top of the pile where steam may be visibly exiting. These vents will be clearly evident, as the material at the exit point will be damp-and usually brown.

Monitor temperature by inserting the probe into the top third of the piles. Inserting it at ground level will typically yield unrealistically cool readings. Remember: This is where the pile is drawing in ambient air.